With the increasing demand for high performance integrated circuits, the techniques of microfabrication have been undergoing continuous development and now include the use of scanning-type electron beam lithography systems, both for producing high quality photolithographic masks and for direct pattern generation. A typical integrated circuit design contains a vast amount of information, particularly if the patterns involved are very finely detailed. For example, it requires 2.times.10.sup.8, 5 .mu.m squares to cover a mask of approximately 50 cm.sup.2. Even at a writing density of only 25%, this would require the drawing of at least 5.times.10.sup.7 individual squares. At typical exposure rates of the order of 1 cm.sup.2 /min., this would result in 12 minutes of beam exposure time alone, notwithstanding other times required for production such as alignment, calibration, data processing and developing, etching, and stripping of resist material from the mask. With these latter times included, typical processing times often exceed an hour. In addition, more detailed patterns which take maximum advantage of electron beam capabilities further increase the time required.
In direct pattern generation, where the electron beam system creates a pattern directly on a chip covered with resist material, the often complicated and time consuming mask-making process is eliminated. However, one of the key economic considerations in an electron beam system in a production environment is the throughput achieved by direct writing relative to a system using a series of masks, especially since direct writing is necessarily a series output process. Hence, time constraints become even more critical in direct pattern generation.
The rate at which a pattern can be exposed (i.e., the exposure time per pixel) is limited by several factors, including the sensitivity of the resist relative to the beam current density, the speed of the table carrying the substrate, the maximum spot size, the ability to deflect the beam at high speed, the data rate of the computer systems, and, in raster scan systems, the blanking time (i.e., the time to turn the beam on and off). Each of these parameters has been the subject of extensive investigations in recent years, and significant advances have been made in each. For example, high sensitivity electron resists are now readily available, computer systems have seen major advances in speed, exotic control systems have been developed to increase speed and to accurately monitor substrate table motion, and beam blanking times have been measurably reduced.
However, as ever higher writing speeds have been sought, other significant problems have also appeared, often as a result of the relationship among these various parameters. For example, as the writing speed increases, the current density must be increased to maintain the same exposure on the resist. However, higher current densities lead to beam broadening due to electron-electron interactions, thereby deleteriously increasing the line width. Also, a shortened exposure time further requires a shortened blanking time, since the rise time of the blanker is closely related to the accuracy of the exposure of each pixel, and is also a major concern in avoiding extraneous exposure during blanking. Hence, blanking time in raster scan type electron beam devices remains one of the key factors limiting throughput.
Most commonly, blankers in these devices consist of two electrostatic deflector plates which deflect the beam off the central axis onto an aperture stop located beyond the deflector plates, thereby turning the beam off. The beam is turned on when the deflector plates are no longer charged, permitting the beam to pass through the aperture. The location of the deflector plates varies from one device to another depending on the particular configuration of the electron beam column. One common configuration is to image the electron source with a condenser lens, and to locate the deflector plates symmetrically relative to the image plane (i.e., at an electron beam cross-over) as in the EBES (electron beam exposure system) developed at Bell Telephone Laboratories. In the limiting case where each electron sees a constant electric field as it traverses the deflector plates, this symmetric arrangement ensures that the writing spot does not moving during blanking, since the undeflected beam cross-over becomes the center of deflection, and is the optical conjugate of the writing spot on the resist. Hence, at low data rates where the blanker rise time can be slow compared to the time for an electron to traverse the deflector plates, the electrons see an essentially constant field and there is very little motion of the writing spot during blanking. In this case, blanker rise time is determined primarily by the required exposure accuracy.
This limiting case is closely approximated by the EBES type systems, where exposure times are generally on the order of 0.025 .mu.sec. to 0.1 .mu.sec. (i.e., a data rate of up to 40 MHz) at beam currents on the order of 20 nA. At an electron energy of 20 KV and deflector plates approximately 4 cm. in length, the transit time for an electron between the deflector plates is of the order of 0.5 nanosecond, while a typical blanker rise time is of the order of 10 nanoseconds to ensure exposure accuracy. Under these conditions, field variations seen by an electron during transit between the deflector plates are of the order of 5%. Hence, there is very little spot motion on the resist during the blanker rise time. Also, with the blanker rise time being short relative to the exposure time, what little spot motion there is does not cause problems with extraneous exposure.
However, it is apparent that at significantly higher exposure rates, the EBES type systems would encounter serious difficulties. For example, an exposure rate of 300 MHz would correspond to exposure times of the order of 3.3 nanoseconds for the same or similar resist, and beam currents of the order 600 nA. Such an exposure time would place extraordinary demands on the blanker system since, at this high data rate, there would be substantial beam motion during the blanker rise time which could cause extraneous exposure. Typically, if the limit on extraneous exposure is set at approximately 1%, the blanker rise time will also be on the order of 1% of the exposure time, corresponding to a 33 picosecond blanker rise time for a 3.3 nanosecond exposure time. Furthermore, the aperture stop in the EBES type systems typically has a pass aperture of approximately 10 times the beam diameter, thereby avoiding charging effects. However, blanking then requires the beam to be deflected a distance on the order of 10 diameters to avoid exposing the resist. For this geometry, a lateral deflection of the beam by 10 beam diameters corresponds to an angular deflection of 10 times the beam full-angle, and requires relatively large electric fields.
Such blanker rise times at the voltages required for blanking on an aperture stop have not heretofore been achieved in raster scan type electron beam lithography devices.